In recent years, as a great deal of interest has been increasingly directed to energy storage technology, efforts and attempts to research and develop batteries capable of implementing such technology have been increasingly undertaken. In this respect, the field of electrochemical devices has been receiving a great deal of attention, and in particular, a lot of interest has been focused on the development of rechargeable secondary batteries.
The secondary battery is a battery making use of electrochemical reaction occurring between electrodes and electrolyte, upon inserting a cathode and anode into an electrolyte and connecting the cathode with the anode. Unlike a conventional primary battery, the secondary battery is a rechargeable battery having repeated usability by recharging energy consumed in electric/electronic products using a battery charger, and therefore is undergoing rapid growth in conjunction with wireless electric/electronic products.
Such secondary batteries may be classified into nickel-cadmium (Ni—Cds) batteries, nickel-hydrogen (Ni—H2) batteries, lithium ion batteries and lithium polymer batteries depending upon kinds of cathode, anode and electrolyte materials to be used, and may also be divided into cylindrical-, square- and pouch-shaped batteries, depending upon morphology thereof.
Materials (such as cathode/anode active materials, binders, electrolytes and current collectors) utilized in the secondary batteries are electrochemically safe under normal operating conditions, for example operating voltage of 2.5 to 4.3 V, operating temperature of −20 to 100° C., and electrically insulated state between the cathode and anode. However, when batteries are subjected to overcharging, heating or short-circuiting by internal or external factors, structural components of the batteries undergo abnormal chemical reactions, thereby resulting in increased internal temperature of the battery and generation of gases. Gases thus generated lead to increased internal pressure of the secondary battery which further accelerates elevation of the battery temperature and gas generation, consequently causing explosion or ignition of the battery.
Therefore, an essential requirement, which should be considered for the development of secondary batteries, is to secure the battery safety. As attempts to secure the battery safety, there may be mentioned a method of disposing safety elements externally of the cell and a method of using materials inside the cell. The former method involves use of elements such as a Positive Temperature Coefficient (PTC) device using change of temperatures, a protection circuit using changes of voltage, and Safety Vent using changes in internal pressure of the battery, whereas the latter method involves incorporation of materials which can undergo physical, chemical or electrochemical changes in response to changes of the internal temperature or voltage of the cell, or blocking of ion transfer by melting of the separator. As specific examples of the latter method, mention may be made of a method using shutdown functions of the separator per se, a method involving incorporation of additives into the electrolyte, a method using coating materials coated on electrode constituent materials, electrodes or separators, and the like. These materials incorporated are designed to exhibit sensitive and rapid response to changes in the internal temperature or voltage of the cell.
Safety elements disposed externally of the cell exert their functions by using changes of temperature, voltage or battery internal pressure and therefore ensure accurate shutdown, but suffer from disadvantages of high costs. On the other hand, the methods of improving safety via incorporation of desired additive materials inside the cell have advantages of simple and convenient installation due to inclusion of the additive materials in the inside of the cell, but suffer from the problems associated with incapability to ensure reliable safety and therefore are not used alone as a measure to offer the battery safety.
As an attempt to achieve the battery safety via changes of the separator at a high temperature, Japanese Patent Publication Laid-Open Nos. 1996-153542 and 2003-243037 disclose a lithium ion battery having improved safety, wherein a cathode and an anode are constructed such that two electrodes are arranged opposite to each other with a separator having a relatively low-melting point therebetween, under no application of an electrode active material to at least one part of the cathode and anode, and therefore the cathode and anode are short-circuited by primary melting of the separator when the internal temperature of the battery elevates. However, fabrication of the battery having such a specific electrode structure suffers from shortcomings such as low productivity, melting of the separator even at an acceptable operating temperature of the battery, thereby rendering the battery useless, and difficulty to induce desired short-circuiting due to adhering of the melted separator to electrodes, thus limiting achievement of the battery safety.
As such, there remains an urgent need in the art for the development of a more efficient technique capable of ensuring the battery safety via the use of the separator.
Meanwhile, due to a small volume, electrochemical devices having a sequential and multi-stacked structure, composed of cathode plates to which cathode active materials are applied, porous separators through which electrolytes can migrate, and anode plates to which anode active materials are applied, are increasingly in demand in compliance with a trend toward miniaturization of electronic products. As a preferred example of such a stacked electrochemical device, Korean Patent Laid-Open Publication Nos. 2001-82059 and 2001-82060, assigned to the present applicant, disclose a stacked and folded electrochemical device. According to theses arts, the stacked and folded electrochemical device has a structure including multi-stacked unit cells of bicells or full cells and a porous separation film interposed between each stacked unit cell, wherein the porous separation film has a unit length which is determined to wrap the unit cells and folds inward every unit length to wrap each unit cell continuously starting from the central unit cell to the outermost unit cell, or wherein the porous separation film has a unit length which is determined to wrap the unit cells and folds outward every unit length to fold each unit cell in a Z-shape continuously starting from the unit cell of a first position to the unit cell of the last position while the remaining separation film wraps outer portions of the stacked cells.
Such a stacked and folded electrochemical device can be easily manufactured, has a structure which is capable of providing efficient use of space, and can maximize the content of the electrode active material so that a highly integrated battery can be implemented.
In such stacked and folded electrochemical devices, the separator, which is one of unit cell constituent elements, and the separation film, which is interposed between unit cells in a particular manner, exert their shutdown functions by thermal behavior at high temperatures, as discussed hereinbefore. However, the separator in the stacked electrochemical cell is likely to cause short-circuiting by shrinkage of the separator per se, before the desired shutdown functions thereof are exerted at high temperatures. Further, fabrication of the porous separator or separation film via uniaxial orientation or biaxial orientation leads to higher probability of short-circuiting due to shrinkage of the separator or separation film.